This text is an introduction to the problem of low frequency electromagnetic monitoring, with particular emphasis on VLF band. All related issues have been addressed: from historical outlook to numerical analysis, from seismology to geotechnics, electronics, information technology. The approach is informative and didactic. This interdisciplinary approach does not imply a problem of skills to the reader, and my goal has been to complete each specific cultural background. Geophysics is a topic where different training paths converge, usually geological or physical. In this case, addressing also technical aspectcts, it is not really possible to setup a guide for a specific professional figure. For this reason, while some topics will be new, others instead will inevitably seem trivial or...